Crack Propagation Question

A few weeks ago I accidentally dropped my droid on some concrete :) Impact location was near the top center, slightly offset. I notice that an initial crack propagated downward about 1 in before making a perpendicular split. Each side of the crack continued along the phone glass heading towards the right and left sides respectively. When the crack got real close to the edge (about 1 cm) it suddenly split as if there were a fork in the road. The end product was a crack that was very close to symmetrical.

My question is, why did the crack propagate in a symmetrical, but very strange shape? Below is my best effort at a ASCII image